Heretofore, optical devices including an optical waveguide and a light-extraction mechanism, and display devices using the optical devices have been known (see, for example, Description of U.S. Pat. No. 5,953,469). Here, the optical waveguide is configured to guide light entered from a first-end surface towards a second-end surface. The light-extraction mechanism is configured to be mechanically in contact with a side surface of the optical waveguide and thereby to extract the light beams from the side surface of the optical waveguide.
In this optical device, the optical waveguide has the first-end surface cut obliquely. The optical waveguide is irradiated with light beams in the direction orthogonal to the obliquely-cut surface, and guides the light beams to the second-end surface while repeatedly causing total reflections of the light beams on the side surface.
When this optical device is formed with too large a cut angle of the first-end surface in an attempt to increase the number of total reflections per unit length, a larger proportion of the incident light beams in the optical waveguide fails to meet the conditions for the total reflections on the side surface and leak out of the optical waveguide.
In addition, an increase of light components incident at a shallow angle with respect to the first-end surface leads to an increase of a Fresnel reflection loss and consequently a reduction of the light beams entering the optical waveguide. Hence, fewer light beams can reach the light-extraction mechanism, resulting in lower light-extraction efficiency. For this reason, there is a certain limit on the improvement in the light-extraction efficiency.